i'm planning to get a kobo to put koreader on, but i'm also curious about other modifications. for example, i don't believe any kobo supports using a bluetooth keyboard, but i've heard that some people do hardware mods to make this possible. are some kobos better than others for hardware modification? what about stuff like replacing a weak battery?

if i didn't care about hackability i'd probably go with the aura one or aura h2o, but stuff like waterproofing seems to conflict with ease of disassembly. what kobos are best suited to fucking around with?

Personally, I think the Aura HD has the most "hackable" features. It still has the internal microSD as well as an external one. It has internal serial test points on its PCB (that's one way to add the hardware part of a blutooth connection). Its battery is easy to replace.
Not sure how the keyboard interface would work though.